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Most Efficient Return from Gilly?


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So assuming you're in a 15km Gilly orbit and want to return to Kerbin, which is more efficient?

a) Burn a direct transfer to Kerbin from Gilly when its on the dark side of Eve?

or

B) Burn towards Eve to get a gravity assist into a transfer to Kerbin?

Argument for option A might be that you're starting in a higher Eve orbit and so will save on delta V not having to burn from a lower Eve orbit. Also, the velocity from Gilly you start with in the correct direction will save you delta V.

Argument for option B is best use of the Oberth effect in where Eve's gravity is much stronger which results in more kick from a "slingshot". Does this benefit outweight the delta V cost to burn back Eve from Gilly?

Edited by Godit
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The most efficient would be to make a small nudge (it will literally take 1 or 2 m/s) to get yourself out of Gilly's orbit and into a 48,000 x 14,000 km orbit of Eve and then make the burn back to Kerbin from there. The Oberth effect will help if you started out below 1000 km but from Gilly's orbital height you'll actually spend less then what it would take from low Eve orbit, even without considering the cost of lowering your orbit.

The real cost though will be the transfer window - I think it comes around every 130 days or so. You should be able to get back to Kerbin from Gilly for about 1400 m/s if you transfer at the right time, but miss it and the cost skyrockets. Since you aren't using the Oberth effect it's probably easiest to push yourself out of Eve's orbit entirely (into an orbit slightly higher then Eve), then use a maneuver node to find the optimum transfer time.

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Checking my past flights if you flew to Eve at the ideal transfer point then unless you spent weeks orbiting Eve or Gilly you should be in position to make a passable transfer back to Kerbin.

Here is a copy of my return flights (Dust 1 and LanfieldTwo). Neither are at the ideal transfer window but they are close enough (first one is 1169 m/s and then some minor correction after, second one is 1254 m/s with a 175 m/s correction after)

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Gilly -> Eve -> Kerbin takes about 10 m/s more delta-V than the approximately 500 m/s for Gilly -> Kerbin, if you ignore the plane change cost.

Thus, if Kerbin will be near the line of nodes (ascending or descending) at Kerbin intercept, then it doesn't matter which way you go,

because you won't need a big plane change burn later on.

If you need to make a significant plane change to return to Kerbin, you may be able to save a lot of fuel by setting up a inclined pass by Eve.

While still near Gilly, adjust your inclination so that you get a kick out of Eve's orbital plane towards Kerbin's orbital plane.

I haven't done the math on this. Your mileage may vary.

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  Dave Kerbin said:
Checking my past flights if you flew to Eve at the ideal transfer point then unless you spent weeks orbiting Eve or Gilly you should be in position to make a passable transfer back to Kerbin.

Here is a copy of my return flights (Dust 1 and LanfieldTwo).

I don't get it. Eve's inclincation always prevented me from ever getting a "single burn" node to work.

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  technion said:
I don't get it. Eve's inclincation always prevented me from ever getting a "single burn" node to work.

From the image it looks like the encounter occurs at the Descending Node so there is no need to make a plane change. He may have built the normal / anti-normal vector into the ejection burn which is much easier to do with a Precision Node mod.

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  Dave Kerbin said:
The most efficient would be to make a small nudge (it will literally take 1 or 2 m/s) to get yourself out of Gilly's orbit and into a 48,000 x 14,000 km orbit of Eve and then make the burn back to Kerbin from there.

Thanks for the answers. Curious as to how you arrived at such an exact distance to orbit Eve 48,000 x 14,000 km. Did you do some calculations or is a a good rule of thumb that I'm not aware of?

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